The present invention relates to a fines removal system for bag houses and particularly relates to bag houses and methods of operating the bag houses for use in forming a hot mix asphalt.
In making hot mix asphalt, there is a mix design for each asphaltic product to be produced. The mix designer, through laboratory, work and guidelines established by governmental authorities, develops the hot mix asphalt mix design to arrive at the most economical and highest performance pavement. In the development of the mix design, the designer specifies the proper amounts of each size aggregate including fines along with the proper amount of asphalt cement to make the ideal mix design for a particular application. In drum mix or continuous mix plants where the aggregates are dried and processed on a continuous basis and mixed with the asphalt cement, it is critical to control the proportions of these materials on a real time basis so that a proper, consistent mix is obtained.
Because of the variation in the transit time through the system for each material, the control system must compensate so that at the final mixing point, all of the materials are delivered in the proper proportions. Causes for the variation in time may be due to the distance each material must travel, since some feed bins are located farther from the mixing point than others. Additionally, aggregates of different sizes move through the drum at different speeds due to the action of the flighting and the flow of burner gases around the aggregates.
In the case of the very fine materials known as fines, filler or dust in the industry, these materials become substantially airborne and leave the dryer unit with the exhaust gases. In this way, they are separated from the balance of the aggregates which continue to progress through the drum to the mixing zone (if a drum mixer or continuous mix unit). These fines are later captured in the exhaust system by the primary collector (if utilized since not all systems have primary collectors) or by the secondary collector which is most often a bag house. The fines going to the bag house are collected on the surface of the filtering media (bags) and remain there until the bag is cleaned.
In state-of-the-art bag houses, the fines are cleaned from the bags while the unit is being operated. The two most popular bag cleaning methods is by pulse jet or reverse air cleaning, although other methods such as sonic or shaker can be utilized. When a bag house is cleaned while the balance of the plant is still in operation, it is necessary to only clean a portion of the bag house at any one time, since if all areas were cleaned at one time, it would temporarily halt the normal operation of the bag house and interrupt the operation of the entire plant. In addition, if all of the bag house was cleaned at one time, there would be no fines discharge during periods of no cleaning followed by a huge surge of fines immediately after commencement of the cleaning cycle. Since in the drum mix or continuous mix process the fines are ideally fed back to the process at a constant rate, these extreme variations in fines flow are unacceptable.
By only cleaning a small portion of the bag house at a time, continuous operation of the plant can be maintained. In a pulse jet bag house, this is accomplished by cleaning a limited number of rows of bags at a given time. In the case of a reverse air bag house, in a like manner, only one section is cleaned at a time. Despite only cleaning portions of the bag house at a time, surging, however, can still occur.
In processes like drum mix or continuous mix asphalt production, it is necessary to return the dust to the mixer at a rate and proportion which is correct at all times for the mix design. This presents a challenge since the flow of fines from the bag house can vary depending on (i) which section of the bag house is being cleaned at a given time, (ii) the sequence of cleaning and (iii) the speed of extraction from the collection hoppers by the hopper conveyor screws or drag. As a result, it is not uncommon for surges in fines to occur at the discharge followed by periods of low or no flow of fines.
In order to solve or minimize this problem, designers have typically added additional equipment to smooth out these surges. One of the most common systems is known as the xe2x80x9cIndiana Fines Systemxe2x80x9d since it was first utilized in Indiana. In this system, which is also depicted in FIG. 1 hereof, the fines are removed continuously (including surges if present) from one end of the bag house 10 by an elevating screw conveyor 11 and deposited into a large enclosed surge hopper or bin 12. On the surge hopper, there is a high level and a low level detector, not shown, used by the operator to monitor the level of fines in the hopper. On the bottom outlet of the hopper is a variable speed, rotary air lock transfer valve 14. To obtain proper performance, the operator will set the speed of the rotary valve for a given production rate and mix design at a speed, as required, to maintain the level of the fines in the hopper 12 between the high and low level points. In this way, the operator can allow for some surging from the bag house, but will maintain nearly a constant flow rate return of fines to the process. As illustrated, the screw conveyor 11 also can carry the fines beyond the hopper if it is full to a waste discharge port 16. This is not a normal condition, but is provided in case the operator fails to remove the fines from the hopper fast enough.
In FIG. 1, it will be appreciated that the dust which is cleaned from the bags at the gas inlet end 18 of the bag house has a shorter transit time to flow to the discharge screw than fines which would be cleaned from the bags at the opposite end of the bag house. This, along with the actual timing of the cleaning cycle, causes the surging of the fines at the discharge. It will also be appreciated that the xe2x80x9cIndiana fines systemxe2x80x9d adds considerable equipment and length to the bag house. In a particular case, it increased the overall trailer length by over twelve feet. Because of this increase in length, structural changes must be made to the bag house also.
In other systems, the designers remove 100% of the dust from the bag house and transport it to a large mineral filler (fines) silo. In doing this, the fines pass through a mass flow meter to measure the amount of fines that are being removed from the bag house on a real time basis. In the plant process controls, this quantity of fines is then subtracted from the total mass of material being processed. On the outlet of the mineral filer silo, there is then equipped an additional metering device which can both control the extraction rate of fines from the silo and measure the amount of fines being extracted. The plant process controls calculates the amount of fines required as dictated by the mix design, and then controls the extraction and returns to process rate to match the required amount. This type of system is expensive to implement, difficult to calibrate, and difficult to maintain.
In accordance with a preferred embodiment of the present invention, there is provided an elongated housing carrying a separator, preferably a plurality of bags depending from a tube sheet, defining plenums on opposite sides of the bags and tube sheet for containing a gas stream with fines and a clean gas stream, respectively. One or more elongated fines collection hoppers extend longitudinally within the housing for receiving the separated fines. A longitudinally extending collection conveyor, preferably a screw conveyor, conveys the separated fines from adjacent opposite ends of the hopper and in opposite longitudinal directions to a location intermediate, preferably centrally of, the collection hopper. A surge hopper for receiving the fines is mounted externally and at one end of the housing. An elevating conveyor extends longitudinally into the housing and has an inlet adjacent said intermediate location for receiving the separated fines from the longitudinally extending collection conveyor and conveying the separated fines to an outlet for discharge into the surge hopper. The surge hopper has a rotating valve at a discharge outlet for combining the separated fines with the coarse fines previously separated from the gas stream for return to the processing plant, e.g., a drum mixer for making asphaltic material or to a silo for storing the fines.
In a particular preferred embodiment of the present invention, a pair of longitudinally extending hoppers are provided in parallel side-by-side relation for receiving the fines separated from the gas stream. In each hopper, there is provided a screw conveyor having reverse flighting for conveying the separated fines from opposite ends of the separator in opposite directions toward the intermediate location. The elevating conveyor comprises a screw conveyor for transporting the fines from the intermediate location to the discharge hopper.
As a consequence of the foregoing construction, the fines are collected from a central point in the bag house which minimizes the variations in transit time of the fines to the discharge location intermediate the opposite ends of the housing. This, in turn, reduces the tendency of the fines to surge. Additionally, the bag house is preferably mounted on wheels and is therefore portable. By locating the inlet to the elevating screw conveyor centrally of the housing, a shortening of the overall length of the bag house trailer is provided with resulting cost savings and affording a more compact system.
In a preferred embodiment according to the present invention, there is provided a bag house for removing fines from a gas stream, comprising an elongated housing, a separator within the housing for separating fines from the gas stream, at least one fines collection hopper within the housing and extending longitudinally of the housing for receiving the separated fines, a longitudinally extending collection conveyor for conveying the separated fines from adjacent opposite ends of the hopper and in opposite longitudinal directions along the hopper to a location intermediate the opposite ends of the hopper and a longitudinally extending elevating conveyor having an inlet adjacent the intermediate location for receiving the separated fines from the longitudinal extending collection conveyor and conveying the separated fines to an outlet for discharge from the housing.
In a further preferred embodiment according to the present invention, there is provided a method of removing fines from a gas stream containing fines and supplied to a bag house, comprising the steps of (a) providing an elongated housing having a separator for separating the fines from the gas stream and an elongated hopper within the housing for collecting the separated fines, (b) conveying the separated fines from adjacent opposite ends of the hopper and in opposite longitudinal directions along the hopper to a location intermediate the opposite ends of the hopper, (c) conveying the separated fines from the intermediate location to a location external to one end of the housing and (d) discharging the separated fines adjacent one end of the housing into a discharge hopper.